The power and other industries require precision measurements of cables and other electrical distribution components for safety, maintenance and other purposes. One such application is the measurement of the performance of the insulation on long haul power cabling, to test that insulation for faults, cracks, water damage and other faults and characteristics. That insulation might be, for instance, cross-linked polyethylene (XLPE) or other material used in medium or high voltage or other electrical distribution lines.
The voltages carried by distribution cables may be generally in the range of several thousand volts (kilovolts, kV) or more, whereas the input voltages accepted by measuring instruments used to test the impedance and other parameters of the cabling may be generally in the low voltage range of 10 volts or less. A conventional way of reducing the applied voltage to make measurements of insulation or other material is therefore to interpose a voltage divider in the test circuit to normalize the voltages on the instrument probes. A representation of a conventional voltage divider is show in FIG. 1. However, manufacturing a high voltage divider that works enough precision to make high-resolution measurements on impedance or other quantities has not been an easy problem to solve.
A general design problem with dividers in the high voltage regime is that combining adequate DC and AC characteristics is difficult. For diagnostic measurements of electrical power equipment, frequencies in the range of direct current to 100 Hz may be of interest. A resistive divider can yield high accuracy at DC, but the accuracy of that type of circuit is reduced at AC. The opposite is true for a capacitive divider. Mixed dividers can be used for both DC and AC, but they have a transition range that gives stability problems
In the past, high voltage circuits, including dividers, have typically employed heavy duty components such as thick film resistors and compressed gas capacitors. Those types of parts have been chosen since they can withstand electric voltages of 20 kV or other high voltage lines. However, that grade of component typically suffers from comparatively inferior phase shift stability, linearity and temperature stability, among other disadvantages. These performance characteristics reduce the precision with which insulation or other samples may be tested using a voltage divider built with such components.
Low voltage components, on the other hand, typically exhibit better phase shift stability, linearity and temperature stability than high voltage counterparts. Low voltage components may include, for example, thin film metal resistors and film capacitors, such as polypropylene capacitors. However, these types of parts are generally not built to withstand the relatively intense electric fields carried by distribution grade electric cables. Overheating, arcing, frequency response and other problems may occur. Without modification, these parts are therefore not good candidates for use in high voltage divider designs.
These and other problems exist in designing a precision high voltage divider.